I'm going to go backwards and start with the wave cycle multiplier.
There is really no reason to use a WCM with the GX. WCM do not alter the frequency that is produced by the generator. All it does is enable the generator to hit frequencies above their native limit.
With the GX, this limit is 40 MHz. So while using say a WCM would allow you to go to 80 MHz, it would come at the cost of waveform resolution. While a WCM of 2 does not really kill waveform resolution, the GX's output is much better than the XM and the output would be altered slightly. Rule of thumb should be to use the smallest WCM necessary to get the job done.
The XM benefits from the WCM in that its upper limit is 5 MHz, and so to hit anything higher we use the WCM to go as high as 25 MHz, but after this the XM looses accuracy and power.
There is one exception to this when it comes to the GX that I can think of, and that is when using the WCM to employ spectrum % like in a Spooky Spectrum Sweep. When using the damped waveforms which requires more than one cycle to describe the waveform, WCM is also necessary to account for how many cycles are in the waveform so that the resulting frequency you are after is maintained.
As for octal harmonics, you are correct but the reason to check it is actually for the reciprocal reason. Since most of the database contains mostly lower frequencies (below 1 MHz), when you do a BFB with the GX, a number or results may be above 1 MHz. If you did a reverse lookup for these frequencies directly, you would get nothing back.
In order to see what the higher frequencies may match in the database based on octal harmonics, you would need to check the include octaves to have Spooky2 do the math.
Then if you got a hit for say 6078880 Hz, you would know that in the database that Alkalosis is a reverse match with frequency 379930 Hz.
We did this for the XM for the same reason, but instead of it being more likely that the hit was above the entire database, the pulse scan range was limited to one octave (76k - 152k) and we were looking to see what may have relevance in another area of the database.
For more details, please check the link: